A semipermeable membrane widely in industrial use involving applications including reverse osmosis, gas separation, and ultrafiltration is one prepared from cellulose acetate by a casting process. Although such a membrane is very highly permeable to water but not to sodium chloride, it deteriorates in performance due to the inherent susceptibility of cellulose acetate to hydrolysis. As a result, the ability to remove solutes such as sodium chloride decreases with time and the life of the membrane is therefore limited. Due to these hydrolytic tendencies, cellulose acetate membranes can only be considered for specific uses wherein conditions leading to hydrolysis are not present. Polymers that are chemically more stable have, therefore, begun to replace cellulose acetate as membrane materials.
Basic considerations with respect to semipermeable membranes useful for ultrafiltration, reverse osmosis, gas separation and the like are that they generally exist in one of two forms: (1) dense, essentially non-porous films and (2) porous, asymmetric films. A dense polymer film in many cases can be given uniaxial or biaxial orientation which increases its mechanical strength and its selectivity as a membrane. Unfortunately, such films, as a rule, offer too much resistance to the flow of desirable permeates and have in most applications been replaced by solvent-cast porous membranes. The latter are specifically made to have a very thin, dense `active` surface supported by a porous substructure. In many cases there exists a gradation of pore sizes throughout the thickness of the porous substructure with the smaller pores being closer to the dense `active` surface. It is the `active` dense surface which rejects some solute components and allows passage of others. This selectivity does not depend on the thickness of the active surface as long as the latter has no pinholes or cracks. The porous substructure constitutes the mechanical support for the thin active surface and should offer as little resistance to the flux of the desirable permeates as possible. This is best achieved when the pores are open and interconnecting and, preferably, longitudinal in shape perpendicular to the active surface of the membrane. Such membranes are obtained by careful solvent casting processes known in the art and described, for example, in U.S. Pat. Nos. 3,133,132; 3,133,137; 3,170,867; 3,567,810; 3,615,024 and 3,884,801.
The prior art methods of preparing such membranes are complicated, slow and require the use of difficult operating conditions. As a result, such procedures are restricted to a limited number of polymers. This restriction arises from the fact that it is difficult to select the casting solvent, additives, temperature of the casting solution and conditions of the environment that will yield membranes in continuous processes and at industrially practical output rates. Even at linear production speeds of only one meter per minute, the flux and porosity of membranes are difficult to control in conventional processes and mechanical properties, due to the lack of provision for molecular orientation, are not optimized.
It is the nature of any solvent casting process that the resultant dense active skin surface of the cast membrane is isotropic. This is true whether the casting process is carried out in one step to simultaneously provide the thin skin and the porous substructure or in two steps wherein a thin skin is applied to a prefabricated porous substructure. There is little opportunity to conduct mono- or biaxial stretching of cast film during the slow, complex solvent casting process steps while maintaining the integrity of the assymetric membrane film structure which is of primary importance.
Previous attempts have been made to provide semipermeable membranes of the polyacrylonitrile and other type polymers which are superior to cellulose acetate in chemical, mechanical, and thermal properties as well as in water permeability. Such attempts were intended to simultaneously provide a membrane with a skin layer and a supporting layer, employing a casting process. Often the membranes obtained did not exhibit satisfactory performance because proper casting conditions were difficult to maintain under industrial operating conditions. It is generally recognized that formation of a membrane having a satisfactory skin layer with such polymer types is a tedious and delicate process when using casting procedures.
A recent patent, U.S. Pat. No. 4,364,759, attempts to solve some of the problems connected with the casting of porous membranes by producing a hollow fiber precursor and coagulating the precursor. The resultant membranes however, because they are not biaxially oriented, do not have the bursting strength and separation selectivity required for many service applications wherein strength and selectivity is a prerequisite.
What is desirable, therefore, is a process for the preparation of porous asymmetric film membranes, which process is not only more expeditious to conduct than a solvent casting process but also can directly provide a molecularly oriented dense active skin surface on top of a supporting porous substrate. The provision for such a process and the resulting product would fulfill a long-felt need and constitute a significant advance in the art.